Sagnac interferometers are used mostly in optical devices for rotation sensing of electrical current sensing. Typically, two optical beams travel in the opposite direction in a closed loop (CW and CCW respectively), and the system is symmetrical in absence of the rotation. If the lights of both beams is mixed and impinge a photo-detector, the photo-detector output is constant, and its value can be set by adding a phase shift in the loop. To match the laser source frequency and the resonant frequency, cavity phase shifters can be used.
Much effort has been expended in research and development into means of reducing both their initial cost and life-cycle costs of Sagnac sensors. In recent years, new technologies have enabled other kinds of sensors that are challenging the dominance of conventional Ring Laser Gyroscopes (RLG) (U.S. Pat. No. 5,371,589 by G. Martin) and Interferometric Fiber Optic Gyroscopes (IFOG) (U.S. Pat. No. 6,801,319 by B. Szafraniec) or electrical current sensors (U.S. Pat. No. 6,831,749 by A. Ohno et al.).
In conventional bulk RLG the counter-propagating waves are traveling in a quadrangular or triangular cavity with the mirrors at the extreme corners that redirecting the beams. The frequency difference between two counter-propagating waves is directly proportional to the applied angular velocity due to the Sagnac effect. It is expressed in terms of interference fringes that depend on the difference of the optical resonant frequencies of two beams. The output of an RLG is the beat frequency of the laser lines circulating in opposite directions, requiring relatively simpler electronic signal processing. The frequency output provides a wide dynamic range in the rotation sensing.
The basic IFOG based on Sagnac interferometer where two lights waves traveling in opposite directions in the coil experience different length, which results un different travel times and a phase differences in the two optical waves. The conventional fiber-coil based IFOG measures the photo-detector signal contains a Fourier line spectra of the modulation frequency that sin- or cos- rotation dependent. The spectrum lines are discriminated by lock-in amplifiers. The closed loop IFOG has an optical feedback element providing a feedback phase shift between CW and CCW optical waves proportional to the electrical output signal. The IFOG has the relatively complicated optical and electronic signal processing needed for retrieval of the Sagnac phase shift from the intensity output.
However, IFOG and RLG are very complex systems that require special fibers, separate modulators, optical modal filters, polarization controllers, etc. The need to overcome ambient-fluctuations and unwanted nonreciprocal phase shifts due to due to backscattering, Faraday effect and nonlinear Kerr effect makes such devices very expensive and bulky.
New technologies are driving the need for batch-producible sensors potentially offering a performance improvement, while geared toward low life-cycle cost, small size, low production cost, and large-volume manufacturing (space applications). The recent trend of scientific research toward microsystems for space applications includes the study, design and fabrication of integrated ring resonator Sagnac sensors, based on semiconductor technology (U.S. Pat. No. 5,325,174 by J. Danko, U.S. Pat. No. 5,872,877 by J. Haavisto, U.S. Pat. No. 6,259,089 by V. Vali et al.). Future improvements are expected in sensitivity and reduction of the size and weight of the sensor. Such miniaturizing a sensing device leads to low weight, compactness, low power consumption while allowing an easy access to greater redundancy and integration that is especially important under hostile environment conditions.
Integrated ring-resonator sensors on Sagnac interferometer (both integrated and not-integrated) can be used, for example, for rotation and magnetic field measurements. Ring resonator cavity (RRG) gyroscopes can be used, for example in spacecraft and satellite applications. Being compact, fully autonomous, highly accurate and uninfluenced by weather conditions they can be almost ideal navigation devices.
The critical gaps, separating the RRC from the tangential waveguides of the sensor, determine the input and output coupling ratios of the cavity, which, in turn, define the magnitude of the finesse, the at-resonance transmittance and, finally, the sensitivity of the device.
In existing integrated ring sensors, while providing the high-density integration potential and small dimensions, make efficient adjustments of cavity parameters a challenging task, limiting their direct implication within various optical systems.
The major disadvantage of the existing ring-resonator sensors based on the following limitations:                The size of the integrated resonator cavity is limited by the size of the substrate (wafer) or minimum bending radius of the waveguide (U.S. Pat. No. 5,872,877 by J. Haavisto, U.S. Pat. No. 6,259,089 by V. Vali et al.). In this case the optimal cavity length to provide a desired accuracy can not be achieved.        Because of the high optical confinement and short coupling distances in RRC, the coupling coefficients are not readily tuned once the sensor is fabricated, especially in an independent fashion. For example, it is difficult to ensure that the two coupling gaps/lengths on two sides of the resonator cavity are matched. The finesse and the extinction ratio of the cavity would be impaired if the coupling factors and cavity phase are not matched to required conditions.        The corresponded layouts for the counter propagated waveguides (CW and CCW) are asymmetrical (U.S. Pat. No. 6,259,089 by V. Vali et al.), making device more sensitive to unwanted nonreciprocal perturbations, including ones caused by possible waveguide optical amplification (loss-compensation).        
The RRC with independently tunable add/drop coupling and phase control were reported previously (U.S. Pat. No. 7,123,800 by A. Kaplan) for general purpose.
The spiral waveguide geometry were reported in previous art without connection to sensing cavities (U.S. Pat. No. 7,133,584 by J. Doan, U.S. Pat. No. 6,922,510 by T. Hatanaka, U.S. Pat. No. 5,953,468 by C. Finnila), or for non-integrated fiber coils (U.S. Pat. No. 6,885,456 by Y. Hashimoto). FIG. 1(a),(b) shows examples of the previous art spiral geometry that includes bends of different radii and right-angle intersections to achieve a long path (cavity) in a compact chip. The consecutive sections of different curvature are shown. The type (b) in FIG. 1 is widely used for the silicon-based lightwave integrated planar circuits. Unfortunately, such fold-back bend cavity can not be used in Sagnac interferometer due to a self-compensated Sagnac effect.
For the Erbium doped folded layouts the modeling of amplification is non trivial since the different radiation losses and distortion of the transverse fields of both the pump and the signal have to be taken into account. It was shown that layouts can be ranked in accordance to their ability to provide maximum gain within a given chip area and an optimum layout based on a spiral containing both straight and curved waveguide sections is identified as an optimal. The potential presence of Erbium in the active region ensures a strong anisotropic optical gain, which is polarization dependent. By emitting TE modes only, active cavity can be polarization selective and, therefore, neither effects of birefringence and coupling between the two polarizations, nor additional polarization noise can be observed.
The two major constraints in design of compact spiral erbium-doped waveguide amplifiers (EDWAs) can be considered: a) minimization of the chip area required for obtaining a predetermined gain, and b) maximization of the gain available from a limited area. The optimum layout usually identified by its ability to provide a maximum gain within a given chip area. The best layout appears to be a rectangular-footprint spiral containing both straight and curved waveguide sections, in which the radii of curvature are equalized everywhere, as far as possible, see FIG. 1,(c). While these two constrains are still valid for the Sagnac sensor applications, there is an additional constrain has to be considered that related to reciprocity of the structure with counter-propagating fields. Thus, a geometrically symmetrical layout of Sagnac sensor is desirable to better compensate for various reciprocal noise-factors in counter-propagating fields.
To summarize, there is a need for reliable RRC-sensor with improved sensitivity and accuracy as well as reduced dimensions and weight.
It is desirable to have a sensor with RRC that has an optimal perimeter-to-area (L/A) ratio (losses versus Sagnac effect). By this means, the highest sensitivity for the given cavity losses can be achieved.
At the same time, it is desirable to have a cavity that takes a minimum space on the wafer, to provide a high fabrication yield and low fabrication cost.
Furthermore, it is also desirable to have a cavity that is geometrically symmetrical, in order to provide an identical optical path for the two counter-propagating optical fields.
Furthermore, it is also desirable that the RRC cavity quality factor can be effectively adjusted accordingly to desirable cavity response, thus, for example, enabling the compensation of cavity couplings mismatch caused by fabrication errors.
Furthermore, it is also desirable that the cavity will include the controllable loss-compensation mechanism, such as in-waveguide amplification.
Electro-optical sensor that possess all mentioned qualities can, potentially, incorporate the advantages offered by Fiber Optic Sagnac Sensors while achieving the high cavity finesse and sensitivity of a Ring-Laser Sagnac Sensors at the cost comparable with some Micro-Electro-Mechanical System (MEMS) sensors.